Wells are desirably drilled into formations bearing oil and gas. These producing formations can occur at different depths and have irregular shapes, making some portions of a producing zone larger at some depths in the wellbore than at others. Occasionally, after a well is drilled and completed, the perforated zone begins to produce marginally or not at all. It is then desirable to be able to tap into the same producing zone in a different depth or location, or to continue the wellbore and/or to deviate in a different direction into a totally different zone to a different depth.
In the past, technology has been developed to allow deviation of wellbores so that when drilling a new well, several lateral bores can be made to improve the overall penetration into the producing zone. While these advances in directional drilling have made it possible to produce multiple lateral bores out of a single bore from the surface, a nagging problem has remained unsolved. The basic problem addressed by the apparatus and method of the present invention is how to make use of an existing completed wellbore by deviating therefrom when the production from the wellbore has become marginal. This has become a problem because in completed wellbores, the tubing string and packer are set in a casing, limiting the access to the perforated zone to begin a deviation from the main wellbore. Typically, casing in the pay zone which plays out is in the order of 41/2"-95/8," with the tubing clearance being as small as about 1.995" I.D. Thus, any tool that is to be placed adjacent the played out perforation in the casing must be delivered to that site through the low clearance of the tubing that is set in place. Various through-the-tubing techniques have been attempted to facilitate the bore through the production tubing. Once such method has been to support a side-tracking tool that is lowered through the tubing below the outlet of the tubing as a guide to a milling machine or tool to create the deviated opening through the casing. This device usually consists of only a sliver of casing which is lowered through the tubing. The sliver, however, provides very little support for the milling tool and, as a result, the milling tool can easily roll off of either side due to the narrow width of the sidetracking apparatus, necessitated by having to pass it through the existing tubing.
Still other processes involve passing through the restriction with an inflatable packer or anchoring assembly and getting this device in the large diameter cavity. A whipstock device is either run concurrently or as a latch on mechanism during a subsequent trip. Unfortunately, the torque resistance and stability of the assembly is marginal due to the large gap which may exist between the anchor and casing I.D. As the inflatable wall is the only connection between the whipstock mandrel and casing wall, the gap determines the amount of rigidity. The larger the gap to bridge, the less rigid is the assembly and more likely is the later drilling assembly to fall off or twist the whipstock. The milling tools could even drill off the whipstock and puncture the inflatable element, resulting in loss of pressure, deflation, and loss of orientation. The hole would most likely be abandoned.
Accordingly, there exists a need for a reliable device and method to allow properly controlled deviations to be started in a completed well which has a packer and tubing string. Not only is there a need to deviate in one direction, but it is desirable in an existing well that has a packer and a tubing string to be able to deviate in different directions in the same elevation as well as at different elevations within the wellbore. The apparatus and method of the present invention also facilitate subsequent further extension of the original wellbore, should that become necessary.